Three-dimensionally ordered macroporous materials or inverse opals are inverse replicates of opals consisting of regularly arranged and uniformly sized spherical void spaces of a few hundred nanometers in diameter surrounded by thin solid walls. High porosity inverse opals of various chemical nature such as silica, titania, metal oxides, metals, and semiconductor materials have been shown to be potentially useful in a variety of applications.
Recently, various synthetic techniques for inverse opals have been reviewed in detail. One of the most commonly used methods to prepare inorganic inverse opals (e.g., SiO2 or TiO2) is the colloidal crystal templating (CCT) where hexagonally packed or patterned layers of monodisperse or binary colloidal particles are used as sacrificial templates.
Techniques reported in the literature for silica inverse opal synthesis have been limited to preparing three-dimensional “layered” structures and there is no report on the preparation of inverse silica opals or the like having three-dimensional spherical geometry. This is so because it is has not been possible to make such silica inverse opals using the layer deposit techniques with latex particles of a few hundred nanometers.
Porous silica particles with large surface areas are widely used in the polyolefin industry to support high activity chromium oxide catalysts for high-density polyethylene or metallocene catalysts for α-olefin polymerization in liquid slurry or gas phase polymerization processes. The performance of olefin polymerization catalysts represented by high catalyst activity and the controllability of particle morphology and polymer properties is critical for the competitiveness of industrial polymerization processes. The effectiveness of heterogeneous olefin polymerization catalysts depends on factors such as chemical composition and structure of a catalyst itself, chemical and physical properties of a support material, and supported catalyst formulation procedure.
One of the intriguing issues concerning silica-supported metallocene catalysts in olefin polymerization is the role of a silica support that is the most widely employed support material. For example, the properties of a silica particle surface influence the formation of various types of active sites of different catalytic activity when active metallocene compounds are immobilized with or without methylaluminoxane (MAO). The morphology and physical properties of silica can also affect the performance of a silica-supported metallocene catalyst. Commercially available silica gel is comprised of randomly linked spherical polymerized primary particles that grow to sizes over 4-5 nm before they coagulate to form the aggregated clusters. The properties of silica gels are influenced by the size and state of aggregation of the primary particles and their surface chemistry. Typical silica particles have surface area of 250-300 m2/g and pore size of around 20-30 nm. Porous silica-supported metallocene catalysts usually undergo complex particle fragmentation and growth process as polymerization progresses.
It is generally believed that the initial particle fragmentation affects the catalyst activity as well as the final morphology of a polymer particle. Often, irregular or incomplete fragmentation of silica occurs and a large fraction of catalyst sites are buried in the solid phase and unavailable for the polymerization. Thus, overall polymerization activity of the silica-supported catalysts is strongly dependent on the effectiveness of particle fragmentation. For the homogeneous fragmentation of the silica support, it is required that active catalytic sites are distributed uniformly on the support surface within micro-pores and that pore size and structure are optimally designed. The catalyst activity data reported in the literature by different authors are often inconsistent even for a chemically identical metallocene catalyst. The reported catalyst activity values are mostly time-averaged (i.e., yield/reaction time) and such data do not represent the true catalytic behavior because the polymerization rate or catalyst activity is strongly dependent on reaction time. It is thought that such discrepancies might be also due to the variations in the actual amount of active transition metal deposited on a support material as well as particle disintegration patterns that affect the availability of the catalyst sites for polymerization.